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Lazyness and forM_
by Marc Busqué 13 Apr '18

13 Apr '18

Hi there! I'm using [selda](https://github.com/valderman/selda) package to work with databases. I'm trying to write a function to generate several tables at once. In the following examples, `categories` and `expenses` are two selda `Table`. The other functions in use and their respective imports should be self-evident: If I do: ``` migrate :: IO () migrate = do dir <- dBDir createDirectoryIfMissing True dir forM_ (categories, expenses) $ withDB . createTable ``` tables are not actually created. However, if I do: ``` migrate :: IO () migrate = do dir <- dBDir createDirectoryIfMissing True dir withDB . createTable $ categories withDB . createTable $ expenses ``` Both tables are actually created. So it seems like the thugs created with `formM_` are actually never executed. Is it so? I'm new with Haskell and it seems strange for me. If it is so, doesn't make it useless `forM_` for IO? Furthermore, I can't reproduce it in ghci. Doing this: ``` ["a", "b"] `forM_` print ``` Actually prints both `"a"` and `"b"`. Thanks for any enlightment. Marc Busqué http://waiting-for-dev.github.io/about/

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EitherT
by mike h 13 Apr '18

13 Apr '18

Hi, I’m trying to write EitherT from first principles and I’m stuck at the first hurdle - Functor. I’m looking for a hint rather than a complete answer :) This is what I have newtype EitherT m a b = EitherT {runEitherT :: m (Either a b)} instance Monad m => Functor (EitherT m a) where ---- fmap :: (a -> b) -> f a -> f b fmap f m = EitherT $ do mv <- runEitherT m case mv of Left _ -> return mv Right rv -> return $ Right (f rv) and here is the compilers view Phrase.hs:31:25: error: • Couldn't match type ‘b’ with ‘a1’ ‘b’ is a rigid type variable bound by the type signature for: fmap :: forall a1 b. (a1 -> b) -> EitherT m a a1 -> EitherT m a b at Phrase.hs:27:5-8 ‘a1’ is a rigid type variable bound by the type signature for: fmap :: forall a1 b. (a1 -> b) -> EitherT m a a1 -> EitherT m a b at Phrase.hs:27:5-8 Expected type: m (Either a a1) Actual type: m (Either a b) • In the expression: return $ Right (f rv) In a case alternative: Right rv -> return $ Right (f rv) In a stmt of a 'do' block: case mv of Left _ -> return mv Right rv -> return $ Right (f rv) • Relevant bindings include rv :: a1 (bound at Phrase.hs:31:19) mv :: Either a a1 (bound at Phrase.hs:28:9) m :: EitherT m a a1 (bound at Phrase.hs:27:12) f :: a1 -> b (bound at Phrase.hs:27:10) fmap :: (a1 -> b) -> EitherT m a a1 -> EitherT m a b (bound at Phrase.hs:27:5) what I think I need to do is fmap over the right value after pulling if out of the monad m by doing mv <- runEitherT m these lines from the compiler are particularly confusing Expected type: m (Either a a1) Actual type: m (Either a b) as I believe f is f:: a1->b So just hints please and I expect I’ll have another duh moment. Thanks Mike

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Re: [Haskell-beginners] EitherT
by 鲍凯文 12 Apr '18

12 Apr '18

Hi, Try to unify the types returned by each of the case alternatives, starting from what the compiler has inferred so far. I’ve made this mistake many times lol. Best, toz On Thu, Apr 12, 2018 at 5:38 AM <beginners-request(a)haskell.org> wrote: > Send Beginners mailing list submissions to > beginners(a)haskell.org > > To subscribe or unsubscribe via the World Wide Web, visit > http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners > or, via email, send a message with subject or body 'help' to > beginners-request(a)haskell.org > > You can reach the person managing the list at > beginners-owner(a)haskell.org > > When replying, please edit your Subject line so it is more specific > than "Re: Contents of Beginners digest..." > > > Today's Topics: > > 1. EitherT (mike h) > > > ---------------------------------------------------------------------- > > Message: 1 > Date: Thu, 12 Apr 2018 11:25:57 +0100 > From: mike h <mike_k_houghton(a)yahoo.co.uk> > To: The Haskell-Beginners Mailing List - Discussion of primarily > beginner-level topics related to Haskell <beginners(a)haskell.org> > Subject: [Haskell-beginners] EitherT > Message-ID: <57AFC9F5-3601-4D18-B860-41352D3F2204(a)yahoo.co.uk> > Content-Type: text/plain; charset=utf-8 > > Hi, > I’m trying to write EitherT from first principles and I’m stuck at the > first hurdle - Functor. I’m looking for a hint rather than a complete > answer :) > > > This is what I have > > newtype EitherT m a b = EitherT {runEitherT :: m (Either a b)} > instance Monad m => Functor (EitherT m a) where > ---- fmap :: (a -> b) -> f a -> f b > fmap f m = EitherT $ do > mv <- runEitherT m > case mv of > Left _ -> return mv > Right rv -> return $ Right (f rv) > > > > and here is the compilers view > Phrase.hs:31:25: error: > • Couldn't match type ‘b’ with ‘a1’ > ‘b’ is a rigid type variable bound by > the type signature for: > fmap :: forall a1 b. (a1 -> b) -> EitherT m a a1 -> EitherT m a b > at Phrase.hs:27:5-8 > ‘a1’ is a rigid type variable bound by > the type signature for: > fmap :: forall a1 b. (a1 -> b) -> EitherT m a a1 -> EitherT m a b > at Phrase.hs:27:5-8 > Expected type: m (Either a a1) > Actual type: m (Either a b) > • In the expression: return $ Right (f rv) > In a case alternative: Right rv -> return $ Right (f rv) > In a stmt of a 'do' block: > case mv of > Left _ -> return mv > Right rv -> return $ Right (f rv) > • Relevant bindings include > rv :: a1 (bound at Phrase.hs:31:19) > mv :: Either a a1 (bound at Phrase.hs:28:9) > m :: EitherT m a a1 (bound at Phrase.hs:27:12) > f :: a1 -> b (bound at Phrase.hs:27:10) > fmap :: (a1 -> b) -> EitherT m a a1 -> EitherT m a b > (bound at Phrase.hs:27:5) > > > what I think I need to do is fmap over the right value after pulling if > out of the monad m by doing mv <- runEitherT m > > these lines from the compiler are particularly confusing > Expected type: m (Either a a1) > Actual type: m (Either a b) > > as I believe f is f:: a1->b > > So just hints please and I expect I’ll have another duh moment. > > Thanks > > Mike > > > > > > ------------------------------ > > Subject: Digest Footer > > _______________________________________________ > Beginners mailing list > Beginners(a)haskell.org > http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners > > > ------------------------------ > > End of Beginners Digest, Vol 118, Issue 6 > ***************************************** >

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May I ask what is the meaning of ch <= 'z' and 'a' <= ch.
by 清羽 10 Apr '18

10 Apr '18

Hello everyone, I have a simple question here Define a function isLower :: Char -> Bool which returns True if a given character is a lower case letter. You can use the fact that characters are ordered, and for all lower case letters ch we have ′a′ ≤ ch and ch ≤ ′z′. Alternatively, you can use the fact that ['a'..'z'] evaluates to a list containing all lower case letters. And May I ask what is the meaning of ch <= 'z' and 'a' <= ch? I copied this as a condition and solved the problem.here is my work. isLower :: Char -> Bool isLower x = if x ch <= 'z' && 'a' <= ch then True else False. Thanks in advance.

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2018 report
by Karim Salah 10 Apr '18

10 Apr '18

hi everyone, I am wondering how can i submit a contribution in the Haskell report. It is stated up here: http://mail.haskell.org/pipermail/haskell-cafe/2018-March/128829.html that i shall send my contribution hcar at haskell.org which i have no clue how to this i will appreciate any help Thanks in advance

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A question regarding the syntax behind the parenthesis and type class.
by 清羽 04 Apr '18

04 Apr '18

this is the original question: Write a function showAreaOfCircle which, given the radius of a circle, calculates the area of the circle, showAreaOfCircle 12.3 ⇒ "The area of a circle with radius 12.3cm is about 475.2915525615999 cm^2" Use the show function, as well as the predefined value pi :: Floating a => a to write showAreaOfCircle. the type signature works for this function is: showAreaOfCircle :: (Show a, Floating a) => a -> a -> String my question is : I don't quite understand the syntax behind the parenthesis and the type class Floating a. Since pi is not a function, and hence the entire function only takes one argument which belongs to the type class Show, why should i put a Floating a in the type signature in the first place?(i come up with this type signature because of the error message and the suggestion stated above) Thanks

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I have a question in designing a 'show' function
by 清羽 04 Apr '18

04 Apr '18

here is the question: Write a function showAreaOfCircle which, given the radius of a circle, calculates the area of the circle, <code>showAreaOfCircle 12.3</code> ⇒ <code>"The area of a circle with radius 12.3cm is about 475.2915525615999 cm^2"</code> my solution is: showAreaOfCircle :: Show a => a -> String showAreaOfCircle x = "The area of a circle with radius" ++ show x ++ "is about" ++ " " + show(pi * x * x) ++ "cm^2" but there is an error • Could not deduce (Num a) arising from a use of ‘*’ from the context: Show a bound by the type signature for: showAreaOfCircle :: forall a. Show a => a -> String at FirstStep.hs:20:1-41 Possible fix: add (Num a) to the context of the type signature for: showAreaOfCircle :: forall a. Show a => a -> String • In the first argument of ‘show’, namely ‘(pi * x * x)’ In the second argument of ‘(+)’, namely ‘show (pi * x * x)’ In the second argument of ‘(++)’, namely ‘" " + show (pi * x * x)’ I had tried to change the type declaration to Show a => Num a => a -> String i guess there must be a problem with pi. Can someone help me with the error message? Great thanks

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Stack and Cabal
by mike h 29 Mar '18

29 Mar '18

Hi, I have dependencies on mtl >= 2.2.2 and containers >= 0.5.11.0 should these go into my .cabal file or into my local stack.yaml? Which is best practice? Can anyone recommend a stack/cabal ‘cheat sheet’? Thanks Mike

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Two type declarations
by Justin Thong 28 Mar '18

28 Mar '18

What is the difference between these two type declarations? The second one is wrong but I can't convince myself why it should be wrong. Is it because *Int *not a constraint class and it is only an instance of one? My curiousity is why #1 can't be written in the form of #2. I apologise if I am using wrong terminology as type, class and constraint class are used with not much distinction. To add context, this problem is to find a function that will find an element by passing in a list and an index argument. elementAt''' ::[a]-> Int ->a -- #1 elementAt''' [] _= error "list is empty" elementAt''' list index | (index < 1) = error "index has to be positive number" | otherwise= list !! (index-1) elementAt'''' ::(Int b)=>[a]-> b ->a -- #2 elementAt'''' [] _= error "list is empty" elementAt'''' list index | (index < 1) = error "index has to be positive number" | otherwise= list !! (index-1) Thank you. I just began learning Haskell. Yours sincerely, Justin *I check my email at 9AM and 4PM everyday* *If you have an EMERGENCY, contact me at +447938674419(UK) or +60125056192(Malaysia)*

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Rounding to 2 decimal places
by Raghav Malik 27 Mar '18

27 Mar '18

Hi, I have a complex double (x :+ y) where 'x' and 'y' are computed using sin and cos functions on an angle expressed in radians. However, the output on the command line (using GHCi) shows up with a lot of decimal places. How can I limit the values of 'x' and 'y' to having only two decimal places? Thanks, Raghav

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